Postcollisional Tectonics And Seismicity Of Georgia

During the Oligocene, marine Tethyan basins were replaced by euxinic basins, which are considered to represent the beginning of syncollisional development between the Arabian and Eurasian plates in Georgia. Ongoing collision during Miocene-Pleistocene times caused inversion of topography such that fold-and-thrust mountain belts of the Great and Lesser Caucasus, and the intermontane foreland basins in between the two mountain belts, were formed where intra-arc and back-arc basins had been. Analysis of seismic sections showing growth strata in intermountain foreland basins indicates that the thrust system in Georgia was active ca. 4 to 3.5 Ma.Beginning in the late Miocene, coeval with molasse deposition in the foreland basins, subaerial volcanic eruptions occurred, characterized by intensively fractionated magma of suprasubduction-type calc-alkaline series from basalts to rhyolites. Outcrops of the magmatic rocks are exposed along the boundaries of the main tectonic units of the region. Pyroclastic rocks of the first volcanic stage (Goderdzi Formation) contain Upper Miocene-Lower Pliocene petrified subtropical wood and other floral remnants. Marine deposits of the Goderdzi Formation are represented by sandy diatomite, which hosts Upper Miocene nanoplankton.In addition to volcanism, earthquakes indicate active tectonics in Georgia. Some of the major earthquakes have proven to be devastating; i.e., the Racha earthquake of 29 April 1991, with Ms = 6.9, was the strongest ever recorded in Georgia. The fault plane solution data for 130 earthquakes show that the territory of Georgia is currently under latitudinal compression, longitudinal extension, and an overall crustal thickening.A complex network of faults divides the region into a number of separate blocks. The boundary zones between these terrains represent maximum geodynamic activity.Three principal directions of active faults compatible with the dominant, near N-S compressional stress produced by northward displacement of the Arabian plate can be distinguished: one longitudinal, trending WNW-ESE or W-E, and two transversal, trending NE-SW and NW-SE. The first group (WNW-ESE), the so-called "Caucasian" strike, is composed of compressional structures, including reverse faults, thrusts, thrust slices, and strongly deformed fault-propagation folds. The transversal faults are also mainly compressional structures, but they contain considerable strike-slip components as well. The tensional nature of submeridional faults is associated with intensive Neogene-Quaternary volcanism in the Transcaucasus. The NESW left-lateral strike-slip faults are the main seismoactive structures in the western Transcaucasus, while right-lateral strike-slip faults are developed in the southeastern Transcaucasus. Considerable shortening and deformation of Earth's crust have taken place via compressional structures, as well as lateral tectonic escape. The geometry of the tectonic features is largely determined by the wedge-shaped rigid Arabian block (indentor) and by the configuration of the oceanic-suboceanic lithosphere (resistant domains) of the eastern Black Sea and south Caspian Sea, all of which cause bending of the main morphological and tectonic structures of the region around the indenting, resistant domains.